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#1
11-07-2010, 08:55 PM
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Inaccurate Odometer
Is it possible to have an accurate speedometer and an inaccurate odometer at the same time? Is there a frictional coupling between the speedometer and the odometer? I have a 1984 300D with a speedometer that appears accurate, but the odometer seems to register lower than it should.
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Tom Evans 69 280SE 91K (wish I still had it) 73 220D 98K 78 300D 134K 78 300D 185K 81 300SD 301K (still have) 82 300SD 321K (still have) 84 300D 268K 84 6.2 Diesel GMC Suburban 225K (a whole different story!) 
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#2
11-07-2010, 09:05 PM
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Yes. Search on odometer repair here. 123 odometers are notorious for slipping due to the fact that the gears are simply pressed onto a knurled shaft.
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1977 300d 70k--sold 08 1985 300TD 185k+ 1984 307d 126k--sold 8/03 1985 409d 65k--sold 06 1984 300SD 315k--daughter's car 1979 300SD 122k--sold 2/11 1999 Fuso FG Expedition Camper 1993 GMC Sierra 6.5 TD 4x4 1982 Bluebird Wanderlodge CAT 3208--Sold 2/13
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#3
11-07-2010, 11:21 PM
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Having just gone through the odometer repair process, I can tell you that it is entirely possible and quite likely that the speedometer works, while the odometer doesn't. The speedometer relies on a pretty robust direct connection from cable to spinning magnet that drives the cup that moves the needle. Unless you take it apart and somehow damage the cup, it's unlikely that the speedometer will fail.
In the odometer, however, the revolutions go through a series of worm gears to arrive at a brass gear. The brass gear drives a shaft from the outside of the sheet metal bracket to the inside. On the inside of that bracket, it all comes down the friction or coupling between that shaft and the infamous pot metal gear. It's the pot metal gear that drives the rest of the digit chain. When the digit chain is turning over with a lot of 9's (say from 199,999 to 200,000) there is a lot of torque on that pot metal gear, and it starts slipping. A couple of threads that really helped me: Odometer problems W123 Odometer Repair Notes Kurt
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- '79 240D - engine swap complete! Engine broken in! 28-31 mpg! Lovin' the ride! - '86 190D (W201-126) - 2.5 NA engine, 5 speed, cloth interior, manual climate controls, 33-34 mpg (sold to forum member).
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